The single stack magazines in the 3904 and 3906 were thought to appeal to police agencies that were "afraid" to issue or authorize the hi cap mag weapons to LEO personnel. In fact several agencies did just that-initially.The 04/06s could be easily modified so their mags would accept one extra round.
But as mentioned above the hi cap mags won the day(fortunately) and the 04/06s went away due to lack of sales.
I have an 04 and I really like it and if I were not hopelessly involved with the Browning HP I would probabbly carry one, the ergonomics are perfect for me.